Field of the Invention
The invention relates to a termination element for a waveguide of a fill level measuring device operating with the transit time method. Furthermore, the invention relates to a fill level measuring device operating with the transit time method having a waveguide comprised of at least one outer conductor and one inner conductor for guiding electromagnetic signals.
Description of Related Art
In a fill level measuring device that operates using the transit time method, the so-called TDR (time domain reflectometry) measuring principle is used in that the transit time of electromagnetic signals led along a waveguide are evaluated. The signals are thereby sent along the waveguide—or, in particular, along at least one of the two conductors forming the waveguide—in the direction of the surface of the medium whose fill level is to be measured. If the signals strike the surface of the medium, they are then partially reflected there. The fill level of the medium can be determined using the transit time between emitting and receiving the reflected signals. The great advantage of guiding the microwave signals is that changing ambient conditions (e.g., increasing or decreasing ambient pressure, increasing or decreasing temperature) do not affect the measuring accuracy and that, additionally, the transit time of the signal is independent of the relative permittivity of the medium.
The conductors, for the most part, have an attachment end, with which they are attached to a connector or flange, and a free end facing the medium. The conductors are connected via the connector to electronics that generate the signals to be transmitted and received as well as evaluate the reflected signals.
In the fill level measurement devices described, so-called “dead zones” exist, which describe the fill level ranges of the medium in which a measurement is not possible or only possible with great inaccuracy. The cause of the “dead zones” lies in impedance jumps of the waveguide used for guiding the signals, which are formed due to mechanical construction or to geometric factors.
Thereby, a difference is made between an “upper dead zone” in the area at which the waveguide is inserted in the container, and a “lower dead zone” at the free end.
In order to deal with the problem of upper dead zones, it is provided in part that the waveguide consists of two conductors, wherein one conductor coaxially surrounds the other conductor. Such coaxial conductors have the advantage of consistent impedance over their geometric course. Additionally, such conductors can, in general, be easily matched to the impedance of the electronics downstream from the waveguide, which is often 50 Ohm.
It is known from the prior art to provide impedance-matching components, termination resistors or impedance-matching networks comprised of different electric components—resistors, capacitors, coils, etc. for the free end of the waveguide. Reference is made, for example, to European Patent Application EP 2 711 673 A1 and corresponding U.S. Patent Application Publication 2014/0084944, German Patent Application DE 100 43 838 A1 or European Patent Application EP 2 154 496 A1. In particular, in European Patent Application EP 2 711 673 A1 and corresponding U.S. Patent Application Publication 2014/0084944, it is provided that an inner conductor is extended through an end piece and that the termination resistor is located in the end piece acting as conductor.
The elements for impedance matching are, thereby, located in the area of the free end of the waveguide, and thus, are subject to process conditions.